Current-limiting fuse



April 8, 1952 w. s. EDsALL x-:T A1. 2,592,399

CURRENT-LIMITING FUSE April 8, 1952 w. s. EDSALI. ET AL CURRENT-LMITINGFUSE 5 Sheets-Sheet 2 Filed Oct. 4, 1949 m10- .Omw NEC' 025m; O

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April 8, 1952 w. s. EDSALI. ET Ax. 2,592,399

CURRENT-LIMITING FUSE Filed Oct. 4, 1949 3 Sheets-Sheet 5 Patented Apr.8, 1,952

William YsfEdsall, Boston, Mass., Kenneth W. Swain, Hampton VVFalls,N.-H., and Frederick-J. Kozacka, Amesbury, Mass., -assignors to TheAChase-Shawmut Company, Newburyport, Mass., a corporation. ofMassachusetts Application October 4, 1949,'Serial No. 119,476

42 Claims.

The' invention refers to fuses of the currentlimiting type which, lwhensubjected to an A.C. iaultcurrent of short-circuit current proportions,'effect interruption of the circuit in less, for evensubstantiallyuless, than half a'cycle of thecurrent wave. Such fuses,when subjected to a D.C.'shortcircuit current having a similar rate`ofvrise as an A'.C.shortclrcuit current, effect :interruptionof theD.'C.current ina similarly `short period of time as in the case oiA.C'.1cur rent interruption. Thelprincipal object of the Sinventionis toprovidea low voltage vcurrent- 'limiting high current interruptingcapacity "fuse Which is improved inveveryrrespect.compared to fuses ofthe same general type whichwere known heretofore.

Current-limiting'fusesare called to interrupt fault currents of'short-'circuit current proportions before the currents reach themaximum `short-circuit current strength which the system or'circuit withwhich the fuses are associated is "capable of producing. lThat currentvis generally referred to as the available short-circuit current. The`maximum current which isV allowedto'ilow ithrough a'circuitf which isprotected' by a currentlimiting -fuse is 'called the let-throughcurrent. Obviously, the let-through current is `much 'smaller thantheavailable short-circuit current. Anycurrent exceeding the let-through'current is considered a short-circuitcurrent.

Current-limiting fuses are often alsorequired `to interrupt overloadcurrents of `much smaller proportions than short-circuit currents if-the overload currents-are of 'relatively long duration. Small overloadcurrents which a current-limiting fuse may beirequired to interrupt maybeinthe order of eight to ten times the rated currentof the fuse.

Originally, -low voltage `current-linliting fuses were relativelycompact on accountof the high heat dissipating'abilityof Vtheirpulverulent ller and on account-of the fact'that the varc energy H2-ndtwhich is dissipated in them is relatively Ain any other circuitinterrupting device. In-

r'creasing concentration of loads made it necessary to providecurrent-limiting fuses of increased current carrying andinterrupting'capacity rating'and the size and bulk of suchcurrentlimiting fuses rhas tended lto increase in propor- .tion to theircurrent carrying and interrupting capacity rating. Fuses for circuitscapable of Vproducing'short-circuit currents in the order of severalhundred thousand amperes are extremely lbulky and diilicult toaccommodate where Yspace limitations are of a serious nature. Itis,therefore, an object of the invention'to provide a low voltagecurrent-limiting-high current interrupting capacity fuse which issmaller and more com- `pactthan prior art current-limiting fuses *of asimilar rating.

The size and bulk lofla current-limiting fuse depend primarily upon theamount of arc energy fz`2r-dt which is Idissipated in the fuse. It wasbelieved, heretofore, that the known designs of current-limiting fusesachieve the aim of minimizing the amount of arc energy which mustnecessarily be dissipated about aswell as that aim can possibly beachieved. It Was found, however, that it is possible to effect afdrasticdecrease in the -amount of arc energy compared to the amount of arcenergy Which is dissipated in current-limiting'fuses Aof known design.In accordance with this invention, a fuse is provided the size and bulkof which -arevreduced in accordance with the newly discoveredpossibility of further reduction of arc energy.

One of the major vdiiiiculties involved inthe design ofcurrent-limitingV fuses is their tendency to behave differentlydepending upon whether the'current under interruption is relatively'highor small. A current-limitingfuse is required'to achieve consistentlysulciently 'rapid interruption irrespective of whether the current underin'- vvterruption is relatively high or small. 'It is, therefore,another object of this invention to'provide a current-limiting Tfusewhich operates satis- Vfactorily both on the occurrence of faultcurrents of short-circuit'current proportions and of relatively smalloverload currents of inadmissible duration.

It has been found that the voltage gradient in a current-limiting fuseis infthe order of 300 volts per centimeter at the instant of arcinitiation and that the voltage gradient at the point of arc initiationdecreases rapidly with increasing arcing time. Because of this decreaseof the voltage gradient at the point of arc initiation, the rate of riseof the total arc voltage-due to progressive Vgrowth of ther length ofthe arc gap on account or continued arcingwill be greatly reduced if thearcing time is not relatively short. It is, there- -iore, anotherobject-ofV this invention to provide a current-limiting fuse in whichVthe arc voltage Yrises so rapidly in case of interruption of relativelysmall overload currents that iinal arc eX- 'tinctionfis eifectedibeforethe voltage` gradient at the point of arc initiation drops considerably.

'Current-limiting fuses which are made according to-this inventioncomprise a fuse link which is arranged in a casingand interconnectsterminal lelements on opposite ends of the casing. The link has aplurality of portions of reduced 'cross-section' spaced' substantiallyequidistantly along substantially the entire length of the link Aand`a'plurality of portions of relatively larger cross-section intermediatethe reduced cross- 'section portions. Thereduced cross-section portionshave a sufficiently small cross-sectional area 'to initiate arcing atinstantaneous currents far below the available short-circuit current ofthe vcircuit or the system with which the fuse is associated. 'Thereduced cross-section portions of the link are veryshprtI and the lengthof theportions of relatively larger cross-section is a multiple of thelength of the reduced cross-section portions. The fuse comprises furthera pulverulent arc-extinguishing substance of predeterminedarc-extinguishing capacity lilled into the casing and surrounding thelink substantially throughout the total length thereof. The substance oriiller has a suiciently high arc-extinguishing capacity in relation tothe arcs which are formed at said portions of reduced cross-section toeffect complete interruption of any short-circuit current that thecircuit with which the fuse is associated is capable of producing inless than 1/120 of a second after fault inception, i. e. less than halfa cycle of a 60 cycle current wave. The total length of said reducedcross-section portions and of said relatively larger cross-sectionportions exceeds the length of the link vaporized under the most onerouscircuit interrupting conditions to which the fuse may be subjected inthe circuit with which it is associated.

Interruption of the short-circuit current by means of such a fuse isinitiated by fusing of the link at the small cross-section portions. Atthe end of the interrupting process,-the small crosssection portions areentirely and the large crosssection portions partially, but notentirely, vaporized, not even under the most onerous circuitinterrupting conditions to which the fuse may be subjected. Such a fuseconsists, after blowing, of a series of spaced bodies of melted sand orfulgurites and intermediate metallic reinainders of the link. Theseremainders of the link are sometimes formed by a more or less solidpiece of metal substantially in its original state with its lengthreduced by arcing and sometimes by a pattern of coarsely dispersed metalpieces.

It is well known in the art to provide fuseI links of low voltage fuseswith portions of reduced crosssection alternating with portions ofrelatively larger cross-section. In a certain type of fuses comprisingsuch links the large cross-section portions, because oi their large heatcapacity, tend to delay the interruption of the circuit by the fuse'incase of protracted relatively small overload currents. The relativelylarge cross-section portions of the link are not involved in theinterruption of large currents in the nature of short-circuit currents.The latter is effected at the portions of reduced cross-section only.Obviously, the present invention has no direct relation to the prior arttime'lag fuses of the aforementioned t-ype. Y

It is also Well known in the art to provide the links or fusibleelements of high voltagey fuses with a plurality of portions of reducedcross-section alternating with portions of relatively largercross-section. These prior art fuses are not of the current-limitingtype and the plurality of portions'of reducedcross-section'of theirlinks is intended to provide a multibreak feature. As a general rule lowvoltage fuses do not call for a multibreak feature. The link of acurrent-limiting fuse is vaporized on the occurrence of a fault currentof short-circuit current proportions suby stantially simultaneously atany point along the entire length of the link, resulting in insertion ofarc resistance into the circuit at a more rapid rate than can beachieved in any prior art high voltage multibreak fuse. In aconventional currentlimiting low voltage fuse, on the occurrence of ashort-circuit current a striation phenomenon occurs throughout theentire length of the link, resulting in the formation ofmultibreakathough the cross-section of the link is uniform throughoutits entire length. This is an automatic multias constant as possible.This, in turn, tends to limit the inductive surge voltages which mayoccur in inductive high-voltage circuits when a current-limiting fuse iscaused to blow and which may present a serious danger to circuitinsulation. In low voltage circuits the danger resulting from inductivesurge voltages is much less serious than in high voltage circuits andcan effectively be eliminated by arranging a plurality of vfuse links inparallel in the circuit which fuse links melt and vaporize sequentiallyunder the action of fault currents of short-circuit current portions.Hence inductive surge voltage control is no reason for providing thelinks of currentlimiting low voltage fuses with a plurality of portionsof reduced cross-section. Though control of inductive surge voltages andthe objects of this invention may both be achieved by means of linkshaving a plurality of portions of reduced crosssection alternating withlarge cross-section portions, either case calls for differentgeometrical link coniiguration. A link having a plurality of reducedcross-section portions designed for inductive surge voltage control willnot achieve the objects of the present invention. Vice versa, a linkhaving a plurality of reduced cross-section portions designed to achievethe objects of the present invention will not achieve inductive surgevoltage control. Inductive surge voltage control requires progressiveinsertion of arc resistance into the circuit. To achieve the objects ofthis invention, the length of the reduced cross-section portions oughtto be Very short in comparison to the length of the large cross-sectionportions. Melting and Vaporization of the former under the action of afault current of short-circuit current proportions results in insertionof but little arc resistance into the circuit. The subsequent meltingand vaporization of the latter results in sudden insertion of a` largeamount of arc resistance into the circuit, which is clearly the oppositeof what is required for inductive surge voltage control.

Both the portions of relatively small crosssection and the intermediateportions of relatively large cross-section of links of prior arthigh-voltage current-limiting fuses are designed to be completelyvaporized at all interruptions of a relatively onerous kind, whereas thelarge cross-section portions of the link of a fuse embodying the presentinvention are designed not to be completely vaporized under the mostonerous circuit interrupting conditions to which the fuse may besubjected in the circuit with which it is associated. The significanceof this relative excess of link length or link metal will now beexplained.

In fuses embodying the present invention the function of the smallcross-section portions and of the large cross-section portions of thelink varies depending upon the magnitude of the current underinterruption.

Currents-.in the'- nature oi` shortfcircuitcurrentsv lrise at'averyrapid ratefand` reach high magnitudes, unless previously interrupted. Infuses'embodying'the present. invention, on oc currence of` ashort-circuitA or.y like fault, the reduced cross-section portions ofthe link reach their fusing temperature. virtually instantly whichlimits the let-throughicurrent to an unusually low value. The adjacentintermediate large cross-section portions ofthe link are still cool atthe time the interrupting process is initiated at the smallcross-section portions. Therefore, the large cross-section portions arecapable of absorbing largeiamounts of heat during the subsequent phaseof the interrupting process during whichthey are partially vaporized.

Currents in the nature of? relatively small protracted overloads resultin a relatively slow rise of the temperature ofthe reduced cross-sectionportions of the link. The rate of rise of the temperature of the reducedcross-section portions is particularly -small Von account `of -theshortness oftheir. length `relative to the length of the largecross-section portions. The heat flow from the reduced cross-.sectionportions to the large'cross-section portions of the link extending overa considerable'period of time prior to initiation of the interruptingprocess results in a more even temperature distribution along the entirelength of the link than in any prior art low voltage current-limitingfuse. Because of that even pre-heating of the entire link before theltime .of` arc initiation, a smaller amount of arcenergyneeds tobedissipated for producing the gap VYreunited for eilecting completeAinterruption ofV the circuitthan is required withoutsuchV anI evenpre-heating of the` entire link length.

YIn the accompanying drawings:

Fig. l is a longitudinal cross-sectionalview of a fuseembodying thepresent invention;

Fig. 2 is a cross-sectional view along line A-A of Fig. 1;

Fig. -3 is a diagram illustrating the` decrease with arcing time of thevoltage gradient at the point of arc initiation;

Fig. 4- is a diagramillustrating the behavior of priorartcurrent-limiting fuses and of current-limiting fuses.` of the presentinvention under short-circuit conditions;

Fig. 5 isa longitudinal cross-sectional View of a' modification offthestructure shown in Figs. 1=and.2;

Fig. 6 shows, in-cross-section, a structure simila-r` to that shownV inFig. 5, however, having a smallerv interrupting, capacity. rating;

Fig. 7 showsin cross-section, a: modification of thestructureshown inFig. 6 particularly de'- signed to effect rapid'interruption of a`circuit after along-time delay-in caseV ofi relatively small overloads;

Fig'. 8v. is a` cross-sectional view on a larger scaleA along B -B ofFig. 7;

9 is a. longitudinal. cross-sectional view of a current-limiting fusehaving.' arnller designed. toevolvea, controlled, limited amount of gas;

Fig. l() shows in front elevation and partially Kinross-section a.composite fuse having a very high current-carrying capacityintended foruse in.A circuits having. very highr available. short-circuitlcurrents;and

Fig. 11- is a cross-sectional' view. alongCe-'C of AFig-10.

In all'gixresathe. same rei'erenoefsigns-fare an plieditorlikezparts; l

VReferring now to Figs.. 1 and 2referenceV numerals; l.' and*- 2ivv areapplied: tothe terminal elementsy or caps aarranged. on opposite. endsof casing,3. 'The fuse. link `4v interconnects the terminalel'ementsIandz2 and mayrbespotwelded to them. The terminal elements I and 2 andthe fuseflinkrare inserted in an electric circuit having a: circuitvoltage notexceeding 600 volts. Thisisthe voltage range for which thefuses according to the present invention are primarily intended; 'I'hecircuit into which the fuse is insertedfhasfbeen indicateddiagrammatically in Fig. 1 byfconductorsiaiand 5b. The fuse shown intFigs. 1` andl2 lends itself for use in A.C. circuits'as'wellaszinzD.-C'; circuits, as will be more fully. explained.' below. Thefuse link `4 has a plurality. of portions. 4a of reduced crossfsectionspaced: substantially eduidistantly along substantially'the entirelengthof link 4 and a plurality ofportions 4b of relatively largercrosssection intermediate said reduced cross-section portions. Eachreduced cross-section portion has-a suiil'cientlyV small cross-sectionalarea to melt andito initiate arcing atr instantaneous currentintensities far. below the available shortcircuit current of thecircuitinto` which thefuse is inserted. The available short-circuit currentoftheicircuit into 'which the `fuse is inserted may, for instanceybeinthe order of 10i peak amperes and arcing'may be initiatedat aninstantaneous current intensity of' i; peak amperes, or 10% oftbefavailable` short-circuitcurrent. Each relativelylargercross-section` portion 4b has a. length w-liiclrl isa multiple off thelength'of'the reduced cross-sectionportions 4a. The casing 3 is lledwith a pulverulent"arc-extinguishingller 6, e. g. cleaniquartz sand,surrounding fuse link 4. The filler 6- has a: suiciently higharc-extinguishing capacity in relationto they number o' points of areinitiation-Which is obviously the same as the` number of portions 4a oflreduced cross-section-to effect extinction of any short-circuit currentarc that the circuit into which the fuse is insertedL is capable ofproducing in less than 1/120 of a second after'the time of faultinception. In other Words, if the fuse is inserted into an A.C.circuithaving a frequency of`60 cycles per second, the time during whicha short-circuit currenty isepermitted to rise plus the arcing time willbe less than half a cycle. If the fuse is designed to interrupt thecircuit with a'minimum of aro energy, the time during which the short;-circuit current is permitted to rise plus the arce ing time will be muchless than half a cycle of the current wave. A fuse according to thepresent invention, when interrupting a short-circuit currentina D.C.circuit, will have an arcing timeanddissipatean amount of arc venergycomparable to,th.e,A.-C. case provided that all other conditions aresimilar. The total length of the reduced cross-section portions 4a andofthe relatively larger cross-section portions 4b exceeds the length oflink 4 which is vaporized under the most onerous circuit interruptingconditions toY which the fuse may be subjected in the circuit into whichit is inserted. Hence, the fuse link 4 will never` be entirelyvaporizecl by arcing. The portions of vthe link which willnot bevaporized even under the most onerous circuit interrupting conditions,i. e. when the amount of arc energy is an optimum, has been indicated inall gures by cross-hatching. The cross-hatched portions of the link mayeither retain their complete integrity after the fuse has blown, or theymay be broken up into a plurality of separated metal parts. Howeverthis'may be, the appearance of the cross-hatched portions of` the linkafter the fuse has blown is very different from the appearance of theother portions thereof since the cross-hatched portions retainsubstantially their metallic nature while the other portions areconverted into fulgurites which are in the nature of glass rather thanthat of a metal.

The required number of portions 4a of reduced cross-section and thetotal length of the reduced cross-section portions 4a and of therelatively larger cross-section portions 4b depends upon such factors asthe circuit voltage, the available short-circuit current and its rate ofrise, the ratio between the cross-sectional area of the reducedcross-section portions 4a and the relatively large cross-sectionportions 4b, the metal of which link 4 is made, the arc-extinguishingcapacity of ller B, the pressure prevailing within casing 3 duringinterruption, etc. Correct data for the required number of reducedcross-section portions lia, and for the total length of the reducedcross-section portions 4a and of the relatively larger cross-sectionportions `4b can readily be determined by power laboratory tests. Suchtests can be greatly simplied by subjecting sets of experimental fusestructures of the type shown in Figs. 1 and 2 to various interruptingconditions and observing in a radioscope the pattern resulting fromblowing of the fuses. If that pattern approaches that indicated in Fig.l, i. e. comprises a number of fulgurites resulting from vaporization ofportions of the link and melting of the surrounding pulverulent lleralternating with more or less metallic portions, this is an indicationthat the fuse is proportioned substantially in accordance with theteachings of the present invention. After having found a relativelyoughapproximation to the required proportions, proportions assuring optimumeffectiveness of the fuse structure may be determined by continuedtesting of varying structures along the established lines.

A point to consider in proportioning a fuse according to the presentinvention may be the inductive voltage surge that is permissible in aparticular circuit. On the basis of a given limitation of thepermissible inductive voltage. surge, the number of portions 4a ofrestricted crosssection may be determined as follows:

Let r be the total arc resistance of a given fuse and rx the arcresistance developed at each point of reduced cross-section of the fuselink. Then v mi; (l) Assuming the fuse is placed in a D.C. circuit whosecircuit voltage is E volts and whose inductance is L henries. Thevoltage equation for such a circuit is A =Er.,i (2) wherein Us is theinstantaneous arc voltage and 1a the instantaneous arc resistance.

Because of the limited insulation level of any electric system, it maybe necessary to impose certain limitations as to the instantaneousvoltage di Ln along 'theinductance In low'voltage systems thepermissible inductive voltage may be in the order of several hundredpercent of the circuit voltage. It may, however, be assumed that arcresistance rs -be introduced into the circuit at such a rate as toinduce exactly the voltage 2E in the inductance L. Then In the aboveequations z' is the current which nows in the circuit at the time tz()of arc initiation. The value rx must be determined experimentally. t maybe necessary to admit higher induced voltages than 2E, and the abovecalculation can be repeated in a similar fashion for any permissibleupper limit of the inductive voltage A fuse structure ofthe type shownin Figs. l and 2 may require special consideration of the problem oflimiting the inductive voltage incident upon blowing of the fuse,particularly in case of a fuse used in circuits involving higher circuitvoltages than the 600 volt range; for which fuses of the present kindare primarily intended. Where a plurality of fuse structures of thepresent invention are arranged in parallel in a low Voltage circuit, thedanger of inductive voltage surges is minimized on account of sequentialblowing of the fuses and that danger hardly needs any consideration.

The operation of a fuse of the type shown in Figs. 1 and 2 is insubstance as follows: On the occurrence of a small but protractedoverload say, in the order of 8 to 10 times the rated current of thefuse, a considerable time is required for increasing the temperature ofthe portions da of reduced cross-section to melting temperature. Thelong time element involved permits a thorough heat exchange between theportions 4a of reduced cross-section and the portions 4b of relativelylarger cross-section. Because of that thorough heat exchange, thetemperature distribution along the entire length of the fuse link 4 will-be relatively even, i. e. the difference between the hottest portionsof the link and the coolest portions of the link will be relativelysmall. In other words, on the occurrence of a relatively small butprotracted overload the fuse link 4 will be thoroughly pre-heated alongits entire length before melting and arc initiation occurs at the pointsof reduced cross-section. Because of the preheating of the portions 4bof relatively largercross-section, a relatively small amount of energyis required after arc initiation at the reduced cross-section portions4a, for converting a certain percentage of the portions 411 into vaporand thus producing the required interrupting gap. The amount of heatstored in the portions 4h prior to arc initiation asaaaea 9.j greatlyreducestheamountof.heat that must be generatedatthe Vportions 4bafterarc initiation to produce the requiredY interrupting Vgap and hencegreatly reducesthearcing. time and the arc energy involvedintheinterruption of overload currents r of relatively small magnitude.

For. a better understanding of4 the operationzof fuses according to thepresent.inventionreierence is made to-Fig. 3.- Figirefers to ay priorart current-limiting low voltage fuseV andshows the voltagegradientatithepointlof arc initiation plotted against time.4 The diagramof.,..Fig. 3 was obtained by impulse testswith substantially constantcurrent. The record starts with a voltage gradient ofV 300 voltsrperrcentimeter and drops within-40 milliseconds to less Ythan.15.voltsper centimeter. The decrease of the voltage gradient and of theresistance of thearc gap at the point of arc initiation with arcing timeis due to progressive heating of the pulverulent: arc extinguishingfiller at this particular point. Such-a decrease of arc resistanceat-the point of arc initiation must be over-compensated by `an increaseof the resistance at other points, i. e. by progressiveV gap elongationresulting from progressive vaporization of the fuserlink.v That processof gap elongation involves relatively long arcingA times and relativelylarge amountsof arc energy'if relatively cold link metal must bevaporized bythe heatof the arc. The process of producing arelativelylarge arc gap involves also -the generation of-relatively large amountsof metal vapor-adverse tO-arc extinction.A Providing a plurality ofportions of reducedcross-section along the fuse link and evenlypre-heating of the entire fuselink before the time 4of arc initiationpermits a reductionof both arcing time and arc energy. Reduction of thearcingtime,V in'turn, means that the circuit is interrupted Ibefore aserious decay of the voltage gradient at the point of arc Vinitiationhas vtaken place.

The amount of metal which is-vaporized in a current-limiting fusedepends,f aside fromthe nature of the pulverulent arc-extinguishingfiller, upon the current density in the fuse-link. The speedatwhich thelength ofthe arc gap increases may be expressed by theV equation'-wherein@ isthe speedof Agrowth of the'gap length, I the rate of currentflow, f the crosssectional area of thelink and C a constanthaving thedimensionrcmA peramp. Thevaluesofthe constant C have been determinedwith various test arrangements andtit was` found that they varied over awide range. Large values oiC. are

obtained where the link is pre-heatedalong.its-

entire length before arc initiation occurs; .whereas small values of Care obtained Where theportions of the link immediately. adjacentthepoint of arc initiation are relativelyY cold. This makes it clear whyunder otherwise similar conditions a link having a plurality of pointsof reduced cross-section uniformly distributed substantially along theentirelength of the link so as to cause an even pre-heating of theentire link in case of relatively small protractedoverloads is conducive to a more rapid growth of the total gap length than a link inwhich relatively cold link metal must be brought up tovaporizationtemperature to produce, thegap length requiredfor finalinterruption of the circuit. The plurality of portions of reducedcross-sectionthus have the cumulative effect of limiting the decay ofthe Kw. seconds Energy' 'required `fcr raising fthe temperature from()rdeg; centtto thefusing point 226 Latent heat' oi melting 1.2 Energy'required for" raising temperature from fusing point' to the boilingpoint.; 3.7 Latentheat'cfvaporization' 23;'0

Total 30:5

Where-small currents are involved, a currentlimiting fuse may beexpectedto work the better, the larger the. portion of the total energyrequiredfor vaporizing'the link is supplied by the circuit prior to arcinitiation and the smaller the portion-of the total energy required forva'- porizing'i-.thelink is suppliedl by the circuit-in formiof areenergy. The geometryY of the fuse link shown in Fig. 1 compliesevidently well with this requirement.

On the occurrence. of' afault current in the nature of ashort-circuitfcurrent, arc initiation is eiected' inv an f extremelyYshort time after inception of the fault; That time is much:l too shortto permit any appreciable pre-heatingv by heatl exchange of portionsA lbby heat generated at the portions 4a. Consequently allthe energy that isneededvfor producing. the required interrupting gap must be furnished informof arc energy by the circuit under interruption. The portions bserve on the occurrenceofffault currents in the nature of short-circuitcurrents as a means for absorbing large amounts of energy at anextremely rapid rate. TheyV and-the ller 6 enable to absorb inmuch lessthan half a cycle of the current wave'all the magnetic en-i ergyinherent in the circuit at the time-of arc initiation.

It appears from the foregoing that the-relef tively'largecross-sectionportions 4b pre-store heat in case of low current circuitinterruptions, thus limiting the amount of arc .energy-.1 that mustbefurnished by the circuit to effectv alcom'zplete interruptionV thereonwhile vthese, portions help to rabsorbthe `large amounts of energy thatare present inthe circuit'at Athe time of'arcLinitia'- tion in the: caseof faults0 in theznatureiof'sh'ortcircuits.

Fig. 4 shows,.diagrainmatically the', behavior of prior artcurrent-limiting fuses and' of 'current-limiting fuses of the present'invention under'l short-circuit conditions. V represents they generatedvoltage plotted versus time. The short'- circuit is assumed to occur atthe time t1. The time elapsingbetween the time of Voltage zero to andthe time t1 of fault inception isfoftenA expressed in electricalvdegrees and-referred toas fault starting angle 0. The fault startingangle 0 hasanimportant eiect upon severity ofv interruption, since itldetermines. theVl displacement of the lshort-circuit current. Thedisplacement will behigh, i. e. the short-circuit current will: behighly asymmetrical, if` the fault starting angle 0 is close to zero.Fig. e refers toa case wherethe short-circuit tends to` ber-followedbyai major current loop indicated by the referenceV sign I. Where thefault is initiated at the peak of. vthe voltage wave, i. e. wherethefaultstartin'g angle 1l is 90, the short-circuit current is fullysymmetrical. When interrupting short-circuit currents maximum severityobtains at such a fault starting angle as to cause melting of the fuselink and initiation of arcing shortly prior to, or at the time when thevoltage wave V passes through its peak. Maximum severity obtains ifarcing is initiated between 70 and 80 degrees on the voltage wave. Fig.4 clearly refers to two cases where arcing is initiated shortly .priorto the time when the voltage wave 'passes through its peak. Incase ofthe prior art current-limiting fuse the current rises up to the peak imax. of the letthrough current. That peak occurs a few electricaldegrees prior to the peak of the voltage wave V. The peak i max. of thelet-through current'is reached at the time ofv arc initiation. In thetime between arc initiation and the subsequent zero of the voltage Waveenough arc resistance is established in the fuse to force the currentdown to zero. The are energy released during the time of initiation ofthe arc and the ti-me of its permanent extinction is an important'severity factor. The total arc energy is determined by the equationwherein Vs is the arc voltage and is the arc current. The total arcenergy may be determined from oscillograms by integrating the product ofarc voltage Va and arc current is over the entire arcing period.

The peak current at the time of arc initiation is one of the mostcharacteristic data determining the behavior of a current-limiting fuseunder short-circuit` conditions. The maximum letthrough current i max.of a fuse of the present invention is shown in Fig. 4 to be quitesmaller than the maximum let-through current i max. of a prior artcurrent-limiting fuse having the same current and Voltage ratings. Thearc extinguishing capacity of the arc-extinguishing means, includingthat of the ller 6 of the fuse of the present invention, are so relatedto the mass of metal of which the fuse link consists that the arcingtime is limited-even under the most onerous short-circuit interruptingconditions to which the fuse may .be subjected in the circuit with whichit is associated-to the range of l to 2 times the time elapsing betweeninitiation of the short-circuit and initiation of arcing. In otherwords, even if the fault occurs at such a fault starting angle 0 as tocause arc initiation close to the peak of the generated voltage, the4time required for permanent extinction of the arc will be within therange of l to 2 times the time elapsing between initiation of the faultand initiation of arcing.

The drastic reduction of the peak of the letthrough current i max., andthe consequent drastic reduction of the a-rcing time and of the arcenergy has been achieved in the fuse of the present invention bydrastically reducing the total amount of link metal and particularly theamount of link metal that must be heated up to the time of melting andarc initiation. This drastic reduction in the amount oi metal that mustbe heated to the melting point to initiate arcing is achieved by theprovision of the extremely short portions 4a. of reduced cross-sectionof the link d. These portions do not, in spite of the smallness of theircross-section, result in la, relatively low current rating of the fusebecause they are eiectively cooled during normal operation of the fuseby the large masses of metal constituted by the immediately adjacentportion 4b of relatively larger cross-section. In other words, thereduced cross-section portions greatly reduce the peak of thelet-through current z" max. without aiecting the normal current carryingcapacity of the fuse. It is possible to minimize the total mass of metalof the link and that present at the reduced cross-section portions 4a tosuch an extent that the resultant reduction of the peak of thelet-through current i max. causes a limitation of the arcing time to 1to 1.5 times the time elapsing between initiation of a short-circuit andinitiation of arcing. By suiiciently reducing the total mass of linkmetal and particularly the mass of metal at the points 4a such alimitation may be caused even where the short-circuit is initiated atsuch a lead angle that initiation of arcing occurs substantially at, orjust before, the peak of the voltage wave V.

When reviewing random oscillograms showing the operation of prior artcurrent-limiting fuses having conventional fuse links it is possible todiscover some interruptions wherein the arc lasts not longer than l to1.5 times the time elapsing between fault initiation and are initiation.In the prior art current-limiting fuses such short arcing times can onlybe observed where the interrupting conditions are not severe, i. e. whenarc initiation occurs relatively close to the zero of the voltage waveV. In fuses of the present invention the arcing time is as short underthe most onerous or severe interrupting conditions as with prior artcurrent-limiting fuses under most favorable or relatively favorableinterrupting conditions. This is due, as previously mentioned, to aproper balance between the arc extinguishing capacity of the arcextinguishing means, particularly the pulverulent ller within the fusecasing, and the peak of the let-through current. Since the arcextinguishing capacity of the arc extinguishing means cannot be greatlychanged-it is mainly determined by the thermal characteristics of thebest grade of quartz sand used as a fuse filler-the peak of thelet-through current had to be brought down by appropriate reduction ofthe mass of metal melted prior to initiation of arcing undershort-circuit conditions to enable shortening of the arcing time andreduction of the are energy beyond the limits attained in prior artfuses. By properly designing the restricted cross-section portions 4a ofthe link 4 the arcing time may consistently be limited to 1 to 1.5 timesthe time elapsing between fault initiation and arc initiation even wherethe fault is initiated at such a lead angle that initiation of arcingoccurs within the range of 20 degrees prior and 10 degrees after thepeak of the voltage Wave V.

In order to control the let-through current to an extent which willeffect a consistent reduction of the arcing time of a high currentcarrying capacity current-limiting low voltage fuse, the amount of metalin the fuse link must be reduced to a minimum by reducing the length ofthe fuse link as much as possible. There is a maximum critical linklength for fuses having a voltage rating Within the limits of 600 and1000 volts. That maximum critical link length is about 2 inches.

In the modiiication of the structure of Figs. 1 and 2 which is shown inFig. 5, each of the reduced cross-section portions is constituted by apair of parallel current paths 4a and 4a. Be-

13 cause of the subdivision of the portions of reduced cross-sectionintoV twov parallel current paths 4a. and 4a", upon melting ofthecurrent paths 4a. and 4a two parallel arclets will be initiated. Itis well known that arcs in parallel are unstable and this tends tofacilitate arc extinction. Because of small differences duetomanufacturing tolerances between the current paths 4a. and 4a, theretends to be a slight difference between the time when arclets areinitiated atv these two points. Similarly. extinction of the arcletsformed at these two points occurs sequentially rather thansimultaneously. Sequential initiation and sequential `extinction of thepair or" arclets formedfat each point of restricted cross-section of thefuse link'r tends to limit the rate of change of current andY thereforeto limit inductive voltage surges incident upon interruption ofrelatively highly inductive circuits.

As shown in Fig. 5, the fuse link` 4 has a plurality of rectangularperforations 8 arranged along the center line thereof. Theseperforations are spaced substantially equidistantly and extendsubstantially along the entire length of the link. Each perforation 8 islaterally bounded by a pair of conductive elements forming two separateparallel current paths 4a', 4a. The cross-section of these parallelcurrent paths` 4a' and 4a" is much less than the cross-section of theportions 4b of the link arranged intermediate the perforated portionsthereof. The length of each intermediate portion 4b is a multiple of thelength of each perforated portion. The total length of the perforatedportions and of the intermediate non-perforatedportionsexceeds thelength of the link which is vaporized under the most onerous circuitinterrupting conditions to which the fuse may besubjected in the circuitwith which it is associated. The portions of the link 4 which remainsubstantially intact, i. e. which are not vaporizedeven if the fuse issubjected to the most onerous interrupting conditions which thecircuitcan produce which the fuse is called to protect are indicated bycross-hatching in Fig.f5. The abovemay also be expressed by saying thatthe serially related arclets which form at the reduced cross-section orreduced width portions of the ribbon-type fuse link never merge into onesingle long arc, this being due to the relative length of the largercross-section or larger Width portions of the link.

As previously mentioned, upon blowing of the fuse under short-circuitconditions, the pulverulent arc extinguishing ller 6 forms a fulgurite,or rather a plurality of aligned fulgurties. In order to obtain a fusestructure which is as compact as possible, the diameter of the casing 3should be substantially equal to the diameter of the fulgurite orfulgurites formed by the filler E upon vaporization of the link underthe action of a current of short-circuit current proportions. In priorart current-limiting fuses the diameter of the casing is generally muchlarger than the diameter of the fulgurite or fulgurites formed Withinthe casing, since it was thought, heretofore, that it is necessary toprovide a large excess of ller, for cooling sufficiently the gaseousproducts of arcing, i. e. a large amount ofV filler not melted undershort-circuit conditions and not converted into a fulgurite. It hasnotonly been found that thisV is noti correct, but that it. isdesirable, for reasons other than that of compactness of design, tolimiti the diameter of the casing substantially to that of the.fulgurite or fulgurites formed upon vaporization of the fuse link underShort-circuit conditions. The larger the diameter of the casing inexcess of the diameter of the fulgurite, the larger the heat dissipatingability of the fuse structure, the larger the amount of heat which mustbegenerated by 121' losses within the fuse to obtain the desiredcurrent-time characteristic. Therefore, in order to minimize 121'losses, the diameter of the casing 3 should only slightly exceed thediameter of the fulgurite or fulgurites which form when acurrent-limiting fuse is subjected to a shortcircuit current.

The casing 3 is preferably made of an organic combustible material, suchas hard fiber. The diameter of the casing 3 should slightly exceed thediameter of the fulgurite or fulguritcs formed upon blowing of the fuse,not more than necessary to preclude substantial scorching ofthe materialof which-the casing 3 is made by the'fulgurite or fulgurites which formwhen the fuse blows. Hard fibre and other organic materials evolve gasesunder the action of heat and gases which may evolve from casing 3 tendto provide a protective layer between thev casing and the fulguriteformed therein, reducing the tendency of the former to bedestroyed bythe heat of the latter.

In a fuse of the type shown in Figs. 1, 2-and 5, where the fuse link 4is provided with a plurality of reduced cross-section portions, thediameter of the fulgurite or fulgurites is inthe order of the distancebetween the reduced cross-section portions of the link. Hence thediameter of casing 3 should be in the order of the distance between thereduced cross-section portions.

Fig. 6 illustrates the features of the invention embodied in acurrent-limiting fuse having a relatively small current rating. Thefusible element has but three portions 4a of restricted crosssection.These portions are formed by circular perforations rather than byrectangular perforations, as shown in Fig. 5.

The fuse illustrated in Figs. 7 andr 8 comprises a link 4 havingveportions 4a.', 4a. of restricted cross-section formed by provision ofiive circular perforations in the fuse link; The perforation situated inthe center of the link 4 is provided with a rivet-like insert I0 as canbest be seen in Fig. 8. Insert l0 consists of a metal having a lowerfusing point than the metal of which the fuse link 4 is made and adaptedto form anV alloy with the metal of which the fuse link 4 is made havinga lower fusing point than the metal of which the fuse link 4 is made.

In a fuse as shown in Figs. l, 2, `5 and 6 the point having the highesttemperature is the zone of restricted cross-section situated midwaybetween the ends of the fuse link 4. It is at this point that arcing isinitiated'at the occurrence of very small overloads of inadmissibleduration. Insert l0, owing to its heat absorbing capacity, delays therate of increase in temperature of the middle portion of the fuse link.When insert i0 melts at a given temperature, its melting results in theformation with metal of'which link 4 is made of an alloy having arelatively low melting temperature. This is conducive to rapid fusion ofthe middle portion ofthe link 4, once insert I0 has reached its meltingpoint.

The insert I0 may consistof asuitableeutectic alloy to achieveinterruption'of' the circuit at a temperature which can be predeterminedwith a high degree of accuracy. However, satisfactory results may alsobe obtained with a tin insert in a link of silver.

In prior art fuses comprising a metal having a relatively low fusingpoint intended to form upon fusion thereof with a high fusing pointmetal a low fusing point alloy, formation of the alloy took generallypiace during normal operation of the fuse rather than at the time whenthe fusing temperature of the low fusing point metal was reached. Such agradual conversion of two metals having different fusing points into analloy having a relatively low fusing point is highly objectionable sinceit results in a gradual change of the current-time characteristic of thefuse. An objectionable change of that kind of the current-timecharacteristic of a fuse is often referred-to as aging The structure ofFigs. '7 and 8 is characterized in that it minimizes, or completelyavoids, aging of the fuse. This is probably due to the fact that thesurface along which there is immediate contact between the two metals isminimized as long as the rivet-like insert l is unfused. The shank a ofrivet-like insert Hl` is probably covered with an oxide film tending topreclude the insert from reacting With the surrounding metal of whichthe fuse link 4 is made. This tendency is increased by the formation ofa small gap between the shank Illa of the rivet-like insert lil and thesurrounding fuse metal. The upset head lb of rivet-like insert l caneffectively be separated and isolated from the adjacent metal of whichthe fuse link i is made by means of a layer il of a suitable fluxingagent, such as rosin. That layer Il prevents interaction of the twometals and formation of an alloy only as long as fusion of insert I hasVnot taken place, but is conducive to extremely rapid formation of analloy when insert Il! ultimatelymelts during a period of a small butprotracted overload.

Sound design of a fuse requires that the amount of heat generatedtherein while it is carrying current of permissible magnitude beminimized, and that whatever heat is'generated therein be dissipated insuch a way as to keep the fuse-reasonably cool while it is carryingcurrent. In high capacity and particularly current-limiting fuses anadditional problem of a thermal nature arises. When such a fuse iscaused to blow by a fault current of short-circuit current proportions,a relatively large amount of heat is generated within the fuse at analmost explosive rate, and therefore the temperature prevailing withinthe fuse tends to be high. If blowing of a fuse by a fault current ofshort-circuit current proportions is preceded by a` smalll pro.- tractedoverload, the temperature of the portion of the fuse tube or casingmidway between its ends tends to be already relatively high before anadditional large amount of heat is released in explosion-like fashion.There isa general tendency, particularly under the above describedconditions, that .fuse `tubes or casings of current-limiting fusesbecome damaged, i. e. charred, shortly after successfulvv circuitinterruption has occurred, when the heat generated during theinterrupting process atv the arcing zone is transferred to the axiallyinner .portion of the fuse tube or` casing, where the temperature alwaystends to be highest. Such damage can be preventedby providing a goodthermal insulation between the axially inner portion of the link and thefuse tube or casing, thus diverting the destructive radial heat flowthat tends 'to occur after blowing of a fuse in a directionlongitudinally of the link and precluding overheating of the centerportion of the fuse tube or casing. This is achieved in the embodimentof the Ipresent invention shown in Fig. 9 by providing differentarc-extinguishing pulverulent fillers I2 and 6 Within the fuse or casing3i. e. gypsum at the axially inner portion and quartz sand at theaxially outer portion. Gypsum has a much smaller thermal conductivitythan quartz sand and thus protects effectively the most endangeredportion of the fuse tube or casing 3 against excessive heating. Quartzsand is a far more effective cooling and deionizin'g medium than gypsumand ensures the currentlimiting action of the fuse. Quartz sand is afulgurite-forming substance which evolves virtually no gas under theheat of the arc, while gypsum is not fulgurite-forming and evolvesrelatively large amounts of gas under the heat of the arc. Thecombination of quartz sand and gypsum or similar gas evolving substancesevolving relatively large amounts of gas under the heat of the arcrenders it possible to control with great accuracy the amount of gasgenerated as well as the amount of gas pressure Within the casing.

It was believed, heretofore, that currentlimiting fuses shouldpreferably not be provided with gas evolving fillers, i. e. fillerswhich evolve appreciable amounts of gas under the heat of the arc,because of the difficulties involved in limiting the pressure within thecasing below dangerous pressure levels, i. e. pressures which involvethe danger of bursting of the casing or fuse tube. To providecurrent-limiting fuses only with quartz sand or similar non-gas-evolvingfillers means to foregothe advantages resulting from the increaseddielectric strength of gases under pressure. The structure of Fig. 9permits to combine effective current-limiting action with controlledgeneration of gas Within the casing or fuse tube 3. The pulverulentfulgurite-forming arc-extinguishing ller 6 surrounds all the reducedcross-section portions 4a of the link but one. The pulverulent gasevolving non-fulgurite forming ller I2 surrounds but one reducedcross-section portion of the link. Preferably llers 6 and l2 areseparated by a pair of insulating barriers I2a.

In addition to precluding charring of the center portion of the fusetube or casing 3 and of causing a limited rise of pressure therein, thearrangement shown in Fig. 9 comprising arcextinguishing fillers I2having different properties, at different points of the fuse tube orcasing 3, has a further important advantage. The fulgurite or fulguritesformed in currentlimiting fuses are relatively good conductors ofelectricity. They lose their conductivity relatively rapidly withdecreasing temperature.v If the rate of decrease of temperature of afulgurite is not sufficiently rapid, a small current will continue toflow through the fulgurite for a short time after extinction of the arcand interruption of the circuit have been achieved. That small,undesirable current is often referred to as the follow current. 'Ihegypsum filler l2 in the structure of Fig. 9 has a relatively smallenergy absorbing capacity, but it provides an effective insulatingbarrier between the fulgurites formed on both sides thereof and thusprecludes the flow of a follow curren while the fulgurites are stillrelatively hot and semiconductive.

It is Well known in the art of current-limiting fuses to surround themiddle or center portion of the .fuse link with a thermal insulator, e.g. glass beads. This is, however, done for controlling the fusingcharacteristics of the link rather than for any of the aforementionedpurposes.

The fuse units shown in Figs. 1, 2, 5 and 6 to 9 have a relatively smallcurrent carrying capacity. These fuse units are intended to be combinedinto a composite current-limiting fuse Where a relatively highcurrent-carrying capacity is required. Such a composite highcurrent-carrying capacity current-limiting fuse is formed by an assemblyof a plurality of identical separate fuse units adapted to be arrangedin parallel in an electric circuit. An embodiment of such an assembly orcomposite fuse is shown in Figs. .and l1. A structure of the kind shownin these figures is capable of carrying continuously 4,000 R. M. S.amps., and if desired even much higher currents.

Referring now to Figs. 10 and l1, the composite fuse comprises twoterminal elements I5 arranged on opposite ends of an outer shell orhousing I6. The terminal elements I5 are in the form of cylindrical'caps integral with 'knife blade contacts a adapted vto lbe inserted ina suitable fuse holder `(not shown). Shell l5 is secured to terminalelements i5 by screws i8 or the like. A plurality of fuse uni-ts of thetype shown in Figs. l, 2, 5 Vand 6 to 9 is arranged within the shell vorhousing I6. These units are adapted te bridge or inter-connect terminalelements l5,

-thus bei-ng Aarranged in parallel in the electric circuit to -beprotected. Each said units 20 comprises a casing ymember 3, terminalscaps I, 2 on opposite -ends thereof, a fuse link 4 having a plurality`of portions 4a of restricted crosssectionakl -area arranged within saidcasing member I3 and a .pulverulent arc-extinguishing filler .1.6-likewise arranged .within casing member 3 and surrounding link '4. Theupper terminal caps 2 0f the fuse units 20 are in .contact With .theupper, and the lower terminal caps l of the fuse units 2-0 are incontact with the lower, Itermi-nal element l5. These fuse units .2B are:grouped in yspaced relation to form a .pattern of concentric shells,I1, Ila, I'lb, etc., to cause the total current flowing through said-fuse :units 2-0 to be unequally distributed among fuse units 20pertaining to different shells I'l, Ila, -I'I-b, etc. This, in turn,causes fusion of the portions a of restricted cross-sectional :area oflinks of Afuses 2d pertaining to different shells Il, I'a, Iib, etc. tooccur at slightly different points of time. These time differentials arehelpful in limiting the rate of change di dt of the total current at thetime the peak of the let-through current is reached, thus limiting themagnitude of possible inductive voltage surges. The arrangement ofindividual fuse units 2'0 in a pattern of concentric shells tends 'alsoto establish a magnetic blowout effect by which the arc stream formed ineach individual fuse unit 2i) is impelled in a lateral direction and its`rate of deionization is increased. This magnetic 'blow-out effect andthe unequal distribution of the total current among the individual 'fuse'units 20 'by which the composite assembly ls constituted may be furtherincreased if the currentpath throughthe holder for the fuse and the fuseis substantially U-shaped, resulting in a magnetic loop action upon thearc current in each individual fuse unit 20. The spaces between thecasings 3 of the individual fuse units 20 are 'lled with a granularinorganic cooling substance I9, such as quartz sand, tending to maintainthe fuses in their shell pattern and providing a safety feature in casethat the casing 3 of any of the individual fuse units 20 shouldaccidentally burst on account of an excess of pressure generatedtherein.

As lpreviously mentioned, current-limiting fuses are generally filledwith quartz sand to the exclusion ,of any arc-extinguishing substancewhich evolves substantial amounts ofgas under the heat of the arc as,for instance, gypsum ror marble powder. The `arrangement shown in Fig. 9of a quartz filler at the axially outer ends of the casing and of agas-evolving filler midway between the ends thereof is a first steptoward an vaccurately pressure-controlled current-'limiting fuse. Thesecond step consists in providing venting means in the nature of asafety valve of utmost simplicity and reliability in combination Awithmeans for precluding any damage, such as 0ccurrence of an electricbreakdown, resulting from the egress of lextremely hotarc products fromthe casing of the individual fuse units 20. In the structure of Figs.1.0 and 11 the terminal caps I and 2 of the individual fuse units 20 arebut relatively loosely secured to the casings .3 of the individual fuseunits. The casing 3 and either one. but preferably both, terminal caps Iand2 of each fuse unit 20 define a small leakage gap therebetween. vThisleakage path permits a restricted escape of hot gases evolved within anycasing 3 to the body of sand I0 by which the individual fuse units aresurrounded. .Such ,a Venting arrangement is obviously much lessvexpensive lto provide than the conventional prior art ventingarrangements in non current-limiting fuses which comprise small ventingholes in their terminal caps. The labyrinth leakage path resulting fromrelatively loosely mounting rthe terminal caps I and 2 on 'the casings 3,is much more effective in cooling of hot gases and in preventing lossof pulverulent arc-extinguishing filler than Yany prior art ventingmeans has been. The provision of the body of Sand I 9 immediatelyadjacent the circular outlet of the safety leakage gap formed betweenthe caps vI and 2 and the casings 3 is of vital importance as evidencedby the occasional formation of a ring of melted or chemcally affectedsand immediately adjacent the circular outlet of the safety leakage gap.

Reference is made to copending application Serial No. 60,328, lfiledNovember 16, 1948, for a more detailed disclosure of the compositestructure of Figs. 10 Aand 1-1.

It willbe understood that, although but a few embodiments `of thisinvention have been shown and described in detail, the invention is notlimited thereto and that the illustrated embodiments may vbe modified orother embodiments made Without departing from the spirit and scope ofthe invention as set forth in the accompanying claims.

It is claimed:

l. In combination an electric circuit having a circuit voltage notexceeding the 600 volt range, a current-limiting fuse arranged in saidcircuit, said fuse comprising a casing, terminal elements on oppositeends of said casing, a fuse link interconnecting said terminal elementsand having a plurality cf portions of reduced cross-section 19 spacedsubstantially equidistantly along substantially the entire length ofsaid link and a plurality of portions of relatively larger cross-sectionintermediate said reduced cross-section portions, each said reducedcross-section portion having a sufliciently small cross-sectional areato initiate arcing at instantaneous currents far below the availableshort-circuiting current of said circuit, each said relatively largercrosssection portion having a length being a multiple of the length ofsaid reduced cross-section portions, a pulverulent arc-extinguishingller Within said casing surrounding said links, said iller having asufficiently high arc-extinguishing capacity in relation to the numberof points of arc initiation to effect extinction of any shortvcircuitarc that said circuit is capable of producing in less than 1/120 of asecond after the time of fault inception, the total length of saidreduced cross-section portions and said relatively larger cross-sectionportions exceeding the length circuit, said fuse comprising a casing,terminal elements on opposite ends of said casing, a fuse linkinterconnecting said terminal elements and having a plurality ofportions of reduced crosssection spaced substantially equidistantlyalong substantially the entire length of said link and a plurality ofportions of relatively larger crosssection intermediate said reducedcross-section portions, each said reduced cross-section portion having asufiiciently small cross-sectional area to initiate arcing atinstantaneous currents far below the available short-circuit current ofsaid circuit, each said relatively larger crosssection portion having alength being a multiple of the length of said reduced cross-sectionportions,l a pulverulent arc-extinguishing ller Within said casingsurrounding said link, said filler having a sufficiently higharc-extinguishing capacity in relation to the arcs initiated at saidportions of reduced cross-section to eiTect complete interruption of anyshort-circuit current that said circuit is capable of producing in lessthan one half cycle of the current wave after the ltime of faultinception, the total length of said reduced cross-section portions andof said relatively larger cross-section portions exceeding the length ofsaid link vaporized under the most onerous circuit interruptingconditions to which said fuse may be subjected in said circuit.

3. In combination an electric circuit, a currentlimiting fuse arrangedin said circuit, said fusev tions Ibeing constituted by a pair ofconductive elements forming separate parallel current paths, each saidrelatively larger cross-section portions having a length being amultiple of the length of said reduced cross-section portions, apulverulent arc-extinguishing nller within said casing surrounding saidlink, said filler having a suiciently high arc-extinguishing capacity inrelation to the arcs initiated at said portions of reduced cross-sectionto effect permanent interruption of any short-circuit current that saidcircuit is capable of producing in less than 1/120 of a second after thetime of fault inception, the total length of said reduced cross-sectionportions and of said relatively larger cross-section portions exceedingthe length of said link vaporized under the most onerous circuitinterrupting conditions to which said fuse may be subjected in saidcircuit.

4. In combination an electric circuit, a current-limiting fuse arrangedin said circuit, said fuse comprising a casing, terminal elements onopposite ends of said casing, a fuse link interconnecting said terminalelements, said link having along the center line thereof a plurality ofrectangular perforations spaced substantially equidistantly andextending substantially along the entire length of the link, each saidperforations being laterally bounded by a pair of conductive elementsforming separate parallel current paths, said elements having asufciently small cross-sectional area to initiate arcing before ashort-circuit current reaches the available short-circuit current ofsaid circuit, the portions of said link situated intermediate saidperforated portions thereof having a length being a multiple of thelength ci said perforated link portions, a pulverulent arc-extinguishingfiller Within said casing surrounding said link, said ller having asuiilciently high arc-extinguishing capacity in relation to the arcsinitiated at said portions of reduced cross-section to eiiect permanentinterruption or any short-circuit current that said circuit is capableof producing in less than 1/320 of a second after the time of faultinception, the total length of said perforated portions and of saidintermediate portions exceeding the length of said link vaporized underthe most onerous circuit interrupting conditions to which said fuse maybe subjected in said circuit.

5. A current-limiting fuse comprising a casing, terminal elements onopposite ends of said casing, a pulverulent arc-extinguishing fillerWithin said casing, a fuse link Within said casing interconnecting saidterminal elements and being surrounded by said iiller, said fuse linkhaving a plurality of portions of reduced cross-section and a pluralityof portions of relatively large crosssection intermediate said reducedcross-section portions, each said relatively larger cross-sectionportions having a length being a multiple of the length of said reducedcross-sections portions, and each said reduced cross-section portionsbeing constituted by a pair of conductive elements forming separateparallel current paths.

6. A high current carrying capacity currentlimiting fuse having avoltage rating of 600 to 1000 volts comprising a casing, terminalelements on opposite ends of said casing, a pulverulentarc-extinguishing iiller within said casing, a fuse link within saidcasing interconnecting said terminal elements and .beng surrounded bysaid ller, said fuse link comprising a plurality of portions of reducedcross-section each being constituted by a pair of conductive elementsforming separatev parallel current paths, said link comprising at lea-stone portion of relatively larger cross-section intermediate said reducedcrosssection portions, said relatively larger cross-section portionhaving a length being a multiple of the length of said reducedcross-section portions.

atea-see 21 thetotal length vof 'sa-id link being approximately twoinches.

"7. -A 'high Acurrent carrying vcapaci-ty cur-rentliiniting fuse havinga voltage rati-ng--of-76U0 to "1000 volts ycomprising-'a casing,terminal elements on opposite 'ends of said casing, -'a 'pulverulentarc-extinguishing iil-ler within said-casing, a fuse link lwithin 'saidcasing interconnecting said terminal elements and being surrounded by`'said filler, 'said link comprising at least-two portions havingrectangular perorations and at least 'one portion of relatively largercross-'sectionbetween said Vperforated portions, the length -of saidrelatively larger cross-section `port-ion #being 1a -multiple 'of thelength of each said `perforated porftions 'andthe tota-l length of saidlink being no vgreater than two inches.

-8. A high --current :carrying capacity currentllimiting fuse having a-voltage rating 'between 600 and 1000vol-ts comprising-'a casing,terminal elements on opposite fend-s of said -casing,a pulverulentarc-extinguishing filler within said -casing, a -f-use link-Within saidcasing-interconnecting "said terminal elements and `being surrounded'-by said ii-ller, said link having along the center than two inches,andthe mass of said"1inkbe`ing lsufficiently small and thearc-extinguishing l-capacity-'of said filler being -suiiic-iently'largeto vlimit 'the -a-rcing time to 1 -to 2 times vthe ltime -e'l'apsin'gbetween -fault inception and arc initiation when said fuse is subjectedto Athe-most severe interrupting conditions of the -circuit forWhich-said fuse is intended.

9. A high current carrying capacity currentlimiting fuse having avoltage rating between 600 and 1000 volts comprising a casing, rterminalVelements on vopposite `ends-o1 `said casing, a pulverulentarc-extinguishing filler -wi-thin said -casing, a fuse link Within saidcasing interconnecting vsaid terminal elements and being sur-rounded 'bysaid iiller, said link having a plurality of por- -i tions of uniformlyreduced cross-section 'and lat least one intermediate portion ofrelatively vlar-ger cross-section, said reduced cross-section -portionshaving a length being a fraction ofthe length of said relatively largercross-section portion, the

l0. In combination an elect-ric circuit, .a current-limiting fusearranged in 'said circuit, said 'fuse comprising Aa casing, terminalrelements on opposite ends of said casing, :a Vfuse linkelnterconnecting said terminal elements 'and havinga Aplurality ofportions of reduced :cross-section, arc-extinguishing `means ofpredetermined 'arcextinguishing capacity including 'a pulverulent fillerwithin said casing surrounding :said link,

l1522 1arc-extinguisliing means `being -suinciently .large to limi-tithefarcing time `ito l Vto f2 Ltimes fthe-time -elaps'ing between Ifaultinception and :.arcsinitiaf-ti'on AWhen-said -fuse is subjected tov the:most severe interrupting conditions which can be `pro'dusaed 'fby ysaidAcircuit.

i111. current-ilimiting *fuse comprising a icasing, terminal elements onopposite ends -oftsaid casing, `a 1fuselink within said casinginterconnecting `'said :terminal elements, :said nfuse 'lin'k having faplurality of portions .of reduced crossfsec'tion Vand va plurality of'portions of relatively :large -cross-'section intermediate saidVYreduced cross-'section por-tions, arc-extinguishing Lmeans vofpredetermined arc #extinguishing .capacity )including a pulverulentiiller within 'fsa'id fca'sing usurrounding said link, the lm'a'ss `oflmetal -.of '.said yli-n'k being Vsuiliciently small "to 'limit the'peakof -the let-throughl 'current to lsufiiciently flow 'values to ena-ble"said arc-@extinguishing means itc 'effect com-plete are extinction in 11501.5 times the-time vfelapsi-ng between initiation of 'a short-circuitland -i-ritiationfef arcin'g -when the short-'circuit lis ini-tiat'ed at.such a 'lead ang-le that initiation vof arc-ing A`occurs substantially'at or just `befoifethe rpeak `of vtlre 'voltage' l-wave.

112. IA current-limiting ruse comprising :a icasing, termina-1 `elementson .opposite ends .-of `said casing, la v-iuse 'link within said'leasing interconlne'cting ,said terminal elements, Vsaid 'fuse -lin-khaving a plurality of portions :of reduced `'crosssec'tion .spaced`substantial-ly lequidistanti-y along substantial-ly itl-ie entirelength -fof said link .and va plurality fof portions of relativelylarger cross- Jsection vinteri-nediate 'said Areduced cross-'sectionportions, the length :of 'each said reduced cross- #section portionbeing a small raction of the length of said 4relativelylargercross-section portons, arc-extingu'is'hing 4means of predeterminedlare#extinguishing capacity including va pulveru- `lent iler '-wi'thinsaid casi-ng `surrounding 'said Vxlink, the mass el? v`said li-nk b'eingfsunciently small 1ra-nge of 2'0rdegrees prior to and yllltlegne'esafter `the 'peak of the generated "voltage,

13. @current-limiting fuse comprising :a tubu. lar casing, terminalelements .on opposite ends 4of-.sai'd acasing, afuse link within fsaidfcasing iintercormecting said terminal elements, fa pulverulentarraextinguish-ing fu'lgurite-forming iller .surrounding lfsai'd link,the diameter of said casing being substantially equal to :the :diameterof tlre .fnlgurite ,formed .by-.said filler upon vapor-ization of 4saidlin-k under .the action of fa `current of short-:circuit currentproportions.

14. .-A current-,limiting fuse -comprising a tubular teasing fof an.organic combustible material, terminal .elements -on opposite ends eof:said dcaszing. fatfu-se lin-k said .casing interconnecting said.terminal elements, said -link having 'a plu- 4,r-.alityofportions pfreduced :cross-section, a Apulseraientarc-'extinguishingrulguriteeforming :till- :er surrounding `said link,the :diameter nf said casing being substantially equal "to :thevdiameter of th'e ffulgurite .formed zup'on 'interruption :of a

the mass of metal of said linkbeingsu'fliciently y,

"Iz-Situada small 'and the arc -extinguishing capacity `vof saidshort-'circuit icurrent and just suiciently Ylarge preclude substantial:scorching by y'ful- `gurite Aof the :material of 'which Nsaid casing:is

Y 15. A current-limiting fuse comprising a tubular casing, terminalelements on opposite ends of said casing, a fuse link within said casinginterconnecting said terminal elements, saidlink having a plurality ofportions of reduced crosssection and a plurality of portions ofrelativelylarger cross-section intermediate said reduced cross-sectionportions, a pulverulent arc-extinguishing fulgurite-forming fillersurrounding said link, the diameter of said casing being in the order ofthe distance'between said reduced crosssection portionsof said link tolimit the size of said casing to the size required by the formation ofavfulgurite upon vaporization of said link under the action of a currentof short-circuit current proportions;

16. A current-limiting fuse comprising a tubular casing of an organiccombustible material, terminal elements on opposite ends of said casing,a fuse link interconnecting said terminal elements and having apluralityof portions of reduced cross-section and a plurality ofportions of relatively larger cross-section intermediate said reducedcross-section portions, a pulverulent arc-extinguishing fulgurte-formingller surrounding said link, the diameter of said casing exceeding thediameter of the fulgurite formed by said ller upon vaporization of saidlink under the action of a short-circuit current only to the extentrequired for precluding scorching of said casing by said iulgurite.

17. A current-limiting fuse comprising a casing, terminal elements onopposite ends of said casing, a metallic -iuse link interconnectingVsaid terminal elements and having a plurality of portions of reducedcross-section and a plurality of portions of relatively largercross-section intermediate said reduced cross-section portions, eachsaid reduced cross-section portions having a hole laterally bounded by apair of current paths in parallel, the length of each said relativelylarger cross-section portions being a multiple of the length of saidreduced cross-section portions, a pulverulent arc-extinguishing llerwithin said casing surrounding said link, and an insert arranged in saidhole of at least one of said reduced cross-section portions. said insertconsisting of a metal having a lower fusing point than the metal ofwhich said fuse` link is made and being adapted to form an alloy withthe metal of which said fuse link is made having a -lower fusing pointthan the metal of `which said fuse link is made.

18. A current-limiting fuse comprising a casing, terminal elements onopposite ends of said casing, a pulverulent arc-extinguishing fillerwithin said casing, a metallic fuse link within said casinginterconnecting said terminal elements and being surrounded by saidfiller, said link having along the center line thereof a plurality ofperforations spaced substantially equidistantly and extendingsubstantially along the entire length of said link, and a rivet-likeinsert arranged in one of said perforations, said insert consisting of ametal having a lower fusing point than the metal of which said fuse linkis made and being adapted to form an alloy with the metal of which saidfuse link is made having a lower fusing point than the metal of whichsaid lfuse link is made.

'19;In a current-limiting fuse the combination of a casing, a pair ofterminal elements arranged on opposite ends of said casing, a fuse linkwithin said casing interconnecting said terminal elements, and anarc-extinguishing Vller within said casing and surrounding said link,said iiller comprising a pulverulent substance having relatively smallheat conductivity arranged in one portion of said casing between saidcasing and a portion of said link and an inert pulverulent substancehaving a relatively large heat conductivity arranged in said casingbetween said casing and the remainder of said link. l

20. In a current-limiting fuse the combination of a casing, a pair ofterminal elements arranged on opposite ends of said casing, aribbon-type multiperforated fuse link within said casing interconnectingsaid terminal elements, and an arc-extinguishing filler within saidcasing, said ller consisting of quartz sand at the axially outer ends ofsaid link in immediate contact therewith and of a pulverulent substancehavingv a smaller heat conductivity thansaid sand at the axially innerportion of said link.

21. In a current-limiting fuse the combination of a casing, a pair ofterminal elements arranged onOpposite ends of said casing, a ribbon-typeinultiperforated fuse link within said casing interconnecting saidterminal elements, and an arc-extinguishing ller within said casing,said ller consisting of a pulverulent fulgurite-forming substanceassociated with the axially outer portions of said link and in immediatecontact therewith and of a pulverulent substance of smaller heatconductivity but larger gas evolving capacity than saidfulgurite-forming substance associated with the axially inner portion ofsaid link.

22. In a current-limiting fuse the combination of a casing, a pair ofterminal elements arranged on opposite ends of said casing, a ribbontypefuse link within said casing interconnecting said terminal elements, andan arc-extinguishing ller within said casing, said iiller consisting ofquartz sand at the axially outer portions of said link and of gypsumpowder at the axially inner portions thereof.

23. In a current-limiting fuse the-combination of a casing, a pair ofterminal elements arranged on opposite ends of said casing, a fuse linkwithin said casing interconnecting said terminal elements, vsaid linkhaving a plurality of relatively short reduced cross-section portionsand a plurality of relatively long large crosssection portionsalternating with said reduced cross-section portions, and anarc-extinguishing nller within said casing, said iiller consisting of apulverulent fulgurite-forming substance at.the axially outer ends ofsaid link and of a pulverulent substance of smaller heat conductivitybut larger gas evolving capacity than said fulgurite forming substancemidway between the axially outer ends of said link.

24. In a current-limiting fuse the combination of a casing, a pair ofterminal elements arranged on opposite ends of said casing, a fuse linkwithin said casing interconnecting said terminal elements, said linkhaving a plurality of relatively short reduced cross-section portionsand a plurality of `relatively long large cross-section portionsalternating with said reduced cross-section portions, a pulverulentfulgurite-forming arcextinguishing iiller within said casing surroundingall said reduced cross-section portions of said link but one, and apulverulent gas-evolving non fulgurite-fcrming arc-extinguishing fillerwithin said casing surrounding but one of said reduced cross-sectionportions.

' .25. In ,a current-limiting fuse the combination of a casing, a pairof terminal caps arranged on opposite ends of said casing, a fuse linkwithin said casing. interconnecting said terminal caps, a pulverulentfulgurite-forming non gas evolving filler within said casingsurroundingV one portion of the length of said link, a pulverulent nonfulgurite-forming gas evolving iiller within said casing surroundinganother portion of the length of said link, a substantially pulverulentcooling substance outside of and surrounding said casing, and meansproviding from the inside of said casing a restricted leakage pathbetween said casing and at least one of said terminal caps to saidcooling substance outside of said casing.

26. In a current-limiting fuse the combination of a casing, a pair ofterminal' caps arranged on opposite ends of said casing, a fuse linkwithin said casing interconnecting said terminal caps, said link havinga plurality of relatively short reduced cross-section portions and aplurality of relatively long large cross-section portions alternatingwith said reduced cross-section portions, a pulverulentfulgurite-forming arc-extinguishing filler within said casingsurrounding all said reduced cross-section portions of saidl link butone, a pulverulent gas-evolving non fulguriteforming arc-extinguishingller within said casing, surrounding but one of said reducedcrosssection portions", a substantially pulverulent inorganic coolingsubstance outside of and surrounding said casing, said casing and atleast one of said terminal caps deiining a small leakage gaptherebetween permitting restricted escape ol gases evolved' from saidgas evolving ller to said cooling substance.

27. A current-limiting fuse comprising a casing, terminal elements onopposite ends of said casing, a ribbon-type metallic fuse linkinterconnecting said terminal elements and surrounded by a pulverulentarc-quenching filler of high cooling capacity, said link having aplurality of substantially equidistant perforations arranged along thecenter line4 thereof extending along a predominant portion of the lengthof. said link and defining a plurality ofserially related pairs ofparallel current paths bounded at each side thereof by one of thelateral edges of said link, and a. metal means arranged in contact withsaid link at the area` o1 one of said plurality of pairs of parallelcurrent paths, said means being,

of a metal having a lower fusing point than the metal of which said linkis made and. adapted toform an alloy with the metal of which said linkis made having a lower fusing point than the metal of which said link ismade, whereby fusing is initiated on relatively small protractedoverloads at a point of said link where one of said pairs of parallelcurrent paths is located.

28; A current-limiting fuse comprising aY casing, terminal elements onopposite ends of said casing, a ribbon-type fuse link of silverinterconnecting said terminal elements and surrounded by a pulverulentquartz ller alongv a predominant portion of the length thereof, saidlink having a plurality of substantially equidistant perforationsextending along substantially the entire length of saidlink and denninga plurality of serially related pairs of parallel current paths boundedat each side thereof by one of the lateral edges of said link, and metalmeans in contact with said link adapted to form an alloy with silverhaving a lower fusing point than silver, said metal means being arrangedat the area of one of said pairs of parallel current paths to causeinitial fusion of said. link attheoccurrence of relatively smallprotracted overloads at said one of said pairs of parallel currentpaths.

29. A current-limiting fuse comprising a casing, terminal elements onopposite ends of said casing, a ribbon-type fuse link of silverinterconnesting said terminal elements and surrounded at least in partby a pulverulent quartz filler, saidlink having a plurality ofsubstantially equidistant perforations-extending along a predominantportion of' the length of said link and deiiningv aplurality of seriallyrelated pairs of parallel current paths bounded at each side thereof byone of the lateral edges of said link, and metal'means in contactwithsaid link adapted to form a silvertin alloy with the silver of whichsaid link is made, said metal means being arranged at the area of one ofsaid pairs of parallel current paths to causev initial fusion of saidlink at relatively small protracted overloads at the point thereof Wheresaid one of saidv pairs of parallel current paths-is located;

80. A current-limiting fuse comprising a casing, terminal elements on.opposite ends off saidcasing, a pulverulent arc-quenching lledv Withinsaid casing, a fuse link within said casing interconnecting saidterminal elements and being surrounded by said ller, said link having aplurality of portions of minimum cross-section spaced substantiallyalong the entire lengthA of said link, a plurality of portions ofmaximum cross-section each alternating with one of said pluralityof'minimum cross-section portions, and` a plurality of portions ofprogressively varying cross-section arranged between said minimumcross-section portions and said maximum crosssection portions, saidplurality of minimum cross-section portions and'said plurality ofprogressively varyingv cross-section portions dening a plurality ofserially related pairs of parallel' cur-rent paths, one of saidplurality of pairs of parallel current paths being associated withfusing point aiecti-ng metal means for causing initial fusion of saidlink at theoccurrence of small protracted overloads only at the point ofsaidlink where said one of said plurality of pairs of parallel currentpaths is-located.

3l. A high current-carryingl capacity currentlimiting: fuse having aVoltage rating of 600- to 1000 vol-ts comprising a casing, a pulverulentarc-- extinguishing ller Within said casing, a fuse link Within saidcasing inter-connecting said terminal elements and being surrounded bysaid ller, said fuse link comprising a plurality of portions of minimumcross-section, a plurality rof portions of maximum cross-section and aplurality of intermediate portions of progressively changingcross-section, said plurality ofv portionsof minimumv cross-sectionportions and said plurality of progressively changing cross-sectionportions forminga plurality of serially related pairs of parallelcurrent paths, the sum total of the length of said plurality ofminimumcross'- section portions being but a small fraction of the' sumtotal of the-length of said'plurality of, maximum cross-section portionsand of the length of said plurality of progressively changing cross.-section portions, the total length of' said link being approximately twoinches, and means for causing initial fusion of said link at atemperature lower than the, fusing. point of the metal of which saidlink is made, said initial fusion causing means being, associated withonly one ofsaid. plurality of pairs of parallel current paths toinitiate fusion of said. link at the point. thereof 27 where said one ofsaid plurality of pairs of parallel current paths is located.

32. In combination an electric circuit, a current-limiting fuse arrangedin said circuit, said fuse comprising a casing, terminal elements onopposite ends of said casing, a fuse link interconnecting said terminalelements and having a plurality of spaced portions of minimumcrosssection, a plurality of spaced portions of maximum cross-section,and a plurality of intermediate portions of varying cross-section, eachsaid minimum cross-section portions having a sufficiently smallcross-sectional area to initiate arcing at instantaneous currents farbelow the available short-circuit current of said circuit and beingconstitutedby a pair of conductive elements forming separate parallelcurrent paths, the sum total of the length of said plurality of maximumcross-section portions and of the length of said plurality ofintermediate cross-section portions being a multiple of the sum total ofthe length of said plurality of minimum cross-section portions, meansassociated with only one of said plurality of minimum cross-sectionportions for causing initial fusing of said link at protracted smalloverloads at a lower temperature than the fusing point o1" the metal ofwhich said link is made, a pulverulent arc-extinguishing filler withinsaid casing surrounding said link, said rlller having a suiiicientlyhigh arc-extinguishing capacity in relation to the arcs initiated atsaid portions of minimum cross-section at fault currents ofshort-circuit current proportions to effect permanent interruption ofany short-circuit current that said circuit is capable of producing inless than 1/120 of a second after the time of fault inception, the sumtotal of the length of said plurality of minimum cross-section portions,said plurality of maximum cross-section portions and said plurality ofintermediate portions exceeding the length of said link vaporized underthe most onerous circuit interrupting conditions to which said fuse maybe subjected in said circuit.

33. In combination an electric circuit, a current-limiting-fuse arrangedin said circuit, said fuse comprising a casing, terminal elements onopposite ends of said casing, a fuse link of silver interconnecting saidterminal elements and having a plurality of substantially equidistantcircular perforations arranged along the center line thereof, saidperforations extending along a predominant portion of the length of saidlink and establishing a plurality of serially related pairs of parallelcurrent paths, each said pairs of parallel current paths having across-sectional area suiliciently small to cause arc initiation atinstantaneous currents far below the available Short-circuit current ofsaid circuit, alloy-forming means associated with one of said pairs ofparallel current paths for lowering the fusing point of said link at thepoint where said one of said pairs of parallel current paths is located,a pulverulent quartz filler within said casing surrounding a suicientportion of the total length of said link to cause arcs initiatedsubstantially simultaneously at the occurrence of fault-currents ofshort-circuit'current proportions at said plurality of serially relatedpairs of current paths to be permanently extinguished in less than 1/120of ya second after fault inception, the sum total of the perforated andnon-perforated portions of said link exceeding the length vaporizedunder the most onerous'circuit interrupting conditions to to leave someresidual non-vaporized link metal within said casing even on theoccurrence of said most onerous circuit interrupting conditions.

34. A current-limiting low-voltage fuse comprising a pair of terminals,a ribbon-type multiperforated fuse link connected to and extendingbetween said pair of terminals, a pulverulent fulgurite-formingarc-quenching substance sur.- rounding said link, and a casingsubstantially of an organic material housing said link and saidsubstance, the distance between said link and the internal wall of saidcasing being approximately equal to the radius of the fulgurite formedby said substance upon blowing of the fuse.

35. In a current-limiting low-voltage fuse ci the type wherein onesingle fuse link is arranged in a casing, the combination of a pair ofterminals, a ribbon-type fuse link connectedY to and extending betweensaid pair of terminals, said link having a plurality of portions ofrelatively small width spaced substantially equidistantly and arrangedsubstantially along the entire length of said link and a plurality ofportions of relatively larger width intermediate said small widthportions, each of said relatively larger width portions having a lengthbeing a multiple of the length of one of said small width portions, apulverulent fulgurite-forming arc-quenching ller 'surrounding said link,a casing substantially of an organic material housing said link and saidiller, and the internal diameter of said casing being approximatelyequal to the diameter of the fulgurite formed by said iller upon blowingof the fuse.

36. A current-limiting fuse having a voltage rating of 600 to 1000volts, comprising a pair of terminal elements, a ribbon-type fuse linkof silver connected to and extending between said pair of elements, saidlink comprising a plurality of portions of minimum width, a plurality ofportions of maximum width, and a plurality of intermediate portions ofprogressively changingwidth, said plurality of portions of minimum widthbeing spaced substantially equidistantly and arranged along apreponderant portion of the length of said link, said plurality ofportions of minimum width and said plurality of progressively changingwidth portions forming a plurality of serially related pairs of parallelcurrent paths, the sum total of the length of said plurality of minimumwidth portions being but a small fraction of the sum total of the lengthof said plurality of maximum width portions and the length of saidplurality of progressively changing width portions, a quartz iillersurrounding said link, a casing substantially of organic insulatingmaterial housing said link and said quartz filler, and the distancebetween said link and the internal wall of said casing beingapproximately equal to the radius of the fulgurite formed by said iilleron occurrence of a short circuit current.

37. A current-limiting fuse having a voltage rating of 600 to 1000 voltscomprising a pair of terminal caps, a ribbon-type fuse link conductivelyconnected to and extending between said pair of caps, said link having aplurality of substantially equidistant perforations aligned along thelongitudinal axis of said link and arranged along a preponderant portionof the length thereof, a body of quartz sand surrounding said link, anda tubular casing having a coaxial external and internal surface, saidinternal surface being of an organic insulating material, said casingsupporting said caps and housing said link and said body of quartz sand,the internal diameter of said housing being in the order of the spacingbetween said perforations and not substantially exceeding said spacing.

38. A current-limiting low-voltage fuse of the type wherein one singlefuse link is arranged in a tubular insulating element, comprising a,pair of terminal caps, a ribbon-type fuse link conductively connected toand extending between said pair of caps, said link having a plurality ofcircular substantially equidistant perforations aligned along thelongitudinal axis thereof and arranged along a preponderant portion ofVthe length thereof, a fusing-point-reducing metal element associatedwith one of said perforations, a body of quartz sand surrounding saidlink to form a fulgurite upon vaporization thereof, and a tubularinsulating element supporting said caps and having an external diametersubstantially equal to the internal diameter of said caps, said tubularelement housing said link and said body of quartz sand and having aninternal wall consisting of an organic insulating material, the averagespacing between said link and said internal wall being approximatelyequal to the maximum transversal growth of said fulgurite resulting inmaximum transversal growth of said fulgurite to immediately adjacentsaid internal wall.

39. A current-limiting low-voltage fuse comprising a pair of spacedterminal caps, a body of quartz sand, a ribbon-type fuse linkconductively connected to and extending between said pair of caps andembedded into said body of quartz sand, said link having a plurality ofsubstantially equidistant perforations aligned along the 1ongitudinalaxis thereof and arranged along a preponderant portion of the lengththereof to form a plurality of serially related pairs of parallelcurrent paths, the spacing between each of said plurality ofperforations being sufficiently large to preclude merger of the seriallyrelated pairs of parallel arclets formed at said pairs of parallelcurrent paths on interruption of short-circuit currents into longerarcs, and a tubular insulating element for housing said body of quartzsand and said link and for supporting said caps on the outer surfacethereof, the inner surface of said insulating element consisting of anorganic iny tively connected to and extending between said pair of caps,and embedded in said body of quartz sand, said link having a pluralityof relatively short substantially equidistantly spaced portions ofrestricted width arranged along substantially the entire length thereofand a plurality of intermediate portions of greatly increased width andof sufficient length to preclude merger of the serially related arcletsformed at said restricted width portions on interruption ofshort-circuit currents into one single long arc, and a tubular elementof insulating material for housing said body of quartz sand and saidlink and for supporting said caps on the external surface thereof, saidtubular housing being of an organic nature at the internal surfacethereof.

41. In combination, an electric circuit having a circuit voltage notexceeding 1000 volts, a current-limiting fuse arranged in said circuit,said fuse comprising a casing consisting substantially of an organicinsulating material, a pulverulent arc-quenching filler in said casing,terminal elements on opposite ends of said casing, a ribbontype fuselink conductively interconnecting said elements and surrounded by saidfiller, said links having a plurality of substantially equidistantlyspaced perforations positioned along the longitudinal axis thereof andprovided along a preponderant portion of the length thereof to form aplurality of serially related pairs of parallel current paths, saidpairs of parallel current paths having a sufficiently smallcross-sectional area to cause initiation of serially related pairs ofparallel arclets at the occurrence of a short-circuit current in saidcircuit before said current can rise to the peak value thereof, and thespacing between said plurality of perforations being sufliciently largeto preclude a merger of said serially related pairs of parallel arcletsinto one single long arc.

42. In combination, an electric circuit having a circuit voltage notexceeding 1000 volts, a current-limiting fuse arranged in said circuit,said fuse comprising an insulating casing, terminal elements on oppositeends of said casing, a ribbon-type fuse link interconnecting saidterminal elements and having a plurality of portions of reduced widthspaced substantially equidistantly along a preponderant portion of thelength of said link and a plurality of portions of relatively largerwidth intermediate said reduced width portions, each of said pluralityof reduced width portions having a sufficiently small cross-sectionalarea to initiate arcing at the occurrence of a short-circuit current insaid circuit before said current can rise to peak value thereof, each ofsaid plurality of relatively larger width portions having a length beinga multiple of the length of said reduced width portions, a body ofquartz sand surrounding said link, the number of said reduced widthportions being suiiciently large and the length of said larger widthportions suiiciently high in relation to the arc-quenching-capacity ofsaid body of quartz sand to effect extinction of the serially relatedarclets formed at said reduced width portions at the occurrence of ashort-circuit current in said circuit before said arclets can merge intoone single long arc, the mass of said link being so small as to limitthe peak of the let-through current of said fuse to sufficiently lowvalues to enable said body of quartz sand to effect extinction of saidarclets in 1 to 1.5 times the time elapsing between initiation of ashort-circuit in said circuit and initiation of said arclets, even ifsaid short-circuit is initiated at such a lead angle that initiation ofsaid arclets occcurs within the range of 20 degrees prior and 10 degreesafter the peak of the circuit voltage.

WILLIAM S. EDSALL. KENNETH W. SWAIN. FREDERICK J. KOZACKA.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,140,953 Cole May 25, 19152,157,906 Lohausen May 9, 1939 2,354,134 Ludwig et al. July 18, 19442,496,704 Fahnoe Feb. 7, 1950

